Thermal grain refinement of maraging steel

ABSTRACT

Coarse grained maraging steel is refined by first heating the steel to a temperature between 1,700* and 1,900* F. and then cooling the heated steel to a temperature below that at which the martensite transformation is completed. A steel having a substantially uniform grain size of ASTM No. 7 is obtained when the heating and cooling steps are repeated a total of three times. This invention relates to the thermal grain refinement of maraging steel. In one aspect it relates to a method for thermally treating a course grained maraging steel so that its grain size is reduced.

United States Patent Saul [54] THERMAL GRAIN REFINEMENT 0F [2| l Appl. No.: 860,399

[52] u.s.c1 ..14s/143,-14s/1'34 51 lnt.Cl. ..C2ld 1/00 581 FieldoiSearch ..14s/134,142,143, 144

[56] References Cited UNITED STATES PATENTS 3,131,097 4/1964 Mantel ..l48/l43X 3,178,324 4/1965 Grangeetal... ...14s/144x 3,341,372 9/1967 Sadowski ..14s/142 3,453,153 7/1969 Tuffnelletal. ..14s/134 OTHER PUBLICATIONS Welding Research, Goldberg, May 1968, pages 1995- 2025 [copy Sci Lib] Feb. 15, 1972 Goldberg, Trans. of ASM, Vol. 6], 1968, pages 26. 27.

29-35 [copy Chem Lib] Floreen et al, Trans. of ASM, Vol. 55, I962 pgs. 5l8-522 Tuffnell et al, Trans of ASM, Vol. 59, i966 pgs. 779-781 Primary ExaminerCharles N. Lovell Attorney-Harry A. Herbert, Jr. and Cedric H. Kuhn 5 7] ABSTRACT substantially uniform grain size of ASTM No. 7 is obtained y when the heating and cooling steps are repeated a total of three times.

size is reduced.

3 Claims, No Drawings THERMAL GRAIN REFINEMENT OF MARAGING STEEL BACKGROUND OF THE INVENTION Maraging steels are iron-base alloys having high nickel and very low carbon contents and generally containing as additional ingredients cobalt, molybdenum, titanium and aluminum. Their introduction in 1959 aroused considerable interest as the steels possessed outstanding properties, particularly as regards structural strength and fracture toughness. The interest was especially great in the aerospace world where there was a need for such a steel in the fabrication of large missile cases.

The alloys are termed maraging" because of their martensitic microstructure when annealed and their extremely high strength on being aged in the martensitic condition. Maraging steels often have a coarse grain structure, either as received from the supplier or as produced in castings, in forgings, by welding, or by overheating in general, e.g., to 2,000-2,200 F. and above. In maraging steels higher values of fracture toughness and improved stress corrosion life are realized for finer grain structure. Also, the finer grain size improves workability of the maraging steels at hot working temperatures. It would be very desirable, therefore, to provide a method whereby the grain size could be reduced.

There are two methods presently in use for refining the grain size of metals and alloys. In the first method the metal or alloy is heated to a temperature at which an allotropic transformation occurs by a nucleation and growth mechanism. The second method involves plastic deformation followed by recrystallization. The first method is not applicable to maraging steels since they undergo an allotropic transformation by a reversible shear mechanism wherein the grain shape is inherited. While the second method is effective in refining all metals and alloys, it is unsuitable for use with finished products.

It is an object of this invention, therefore, to provide a new method for the grain refinement of maraging steels.

Another object of the invention is to provide a method for decreasing the grain size of finished products that have coarse grains resulting from overheating during their fabrication.

Still another object of the invention is to improve the workability of maraging steels.

Other and further objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.

SUMMARY OF THE INVENTION Broadly speaking, the present invention resides in a method for thermally treating coarse grained maraging steels so as to obtain a product having a finer grained structure. The method comprises the steps of l) heating a coarse grained maraging steel having a martensite structure to a temperature at which grain stress induced by phase transformation from a martensite to an austenite structure is relieved by grain nucleation and then (2) cooling the heated steel to a temperature below that at which magnetic martensite transformation is completed. The steel product thus obtained has a structure in which the grains are smaller in size than the grains of the starting material.

ln carrying out the method of this invention, the temperature to which the steel is heated, i.e., the refining temperature, is a critical temperature. Heating to temperatures below and above the refining temperature does not result in a decrease in the grain size of the maraging steel. The refining temperature is above the transformation temperature at which the martensite structure of maraging steel is transformed to an austenite structure. It was completely unexpected when it was discovered that there is a critical temperature above this transformation temperature at which reduction in grain size is obtained. Prior to this invention it had been generally accepted that grain size does not change as a maraging steel passes upon heating through phase transition from a martensite structure to an austenite structure and thence upon cooling back to a martensite structure. Contrary to this accepted theory, it was found that the grain stress induced by the transformation cycle is relieved at the critical temperature by the grains nucleating and growing new, finer grains. Thus, the critical temperature can be defined as that temperature at which grain stress in-' duced by phase transformation from a martensite to an austenite structure is relieved by grain nucleation. The refining temperature varies with the different grades of maraging steels, but it generally falls in the range of about 1,700 to l,900 F.

The rate at which the maraging'steel is heated to the refining temperature is not significant. However, it is very important that the steel be heated to the refining temperature, neither below nor above this temperature, for otherwise the desired reduction in grain size will not be realized. Heating should be carried out so that the steel is at a uniform refining temperature. With large specimens very careful control of the heating is necessary in order to ensure that a product having grains of a uniform size is obtained. Thus, while small specimens may be heated to the required refining temperature in a few minutes, extended heating periods, e.g., up to several hours, may be required for large specimens. The cooling step should preferably commence as soon as a uniform refining temperature is attained. By proceeding in this manner the possibility of overheating a specimen is minimized. Thus, it is usually desirable to begin cooling 1 to 10 seconds after a specimen reaches a uniform refining temperature. However, it should be understood that the period prior to cooling can be of any length so long as the specimen is maintained at the refining temperature and is not allowed to become overheated.

After heating to the refining temperature, the steel is cooled to below that temperature at which it transforms completely to the magnetic martensitic phase. This temperature also varies with the particular grade of maraging steel being treated, but it usually falls in the range of to 400 F. The rate and method of cooling do not have an appreciable effect upon the degree of grain refinement. Thus, either slow or rapid cooling can be employed as by cooling in air or by quenching in water or oil.

Maraging steels are characterized by containing a high percentage of nickel together with smaller amounts of cobalt, molybdenum, titanium and aluminum. The steels generally have the following composition expressed in weight percent:

Percent Nickel l5-26 Cobalt 7-[3 Molybdenum 2.75-5.5 Titanium 0.7-l .6 Aluminum 0.05-0.14 Carbon 0.03 [Maximum] Silicon 010 (Maximum) Manganese 0.10 (Maximum) Sulfur 0.0! (Maximum) Phosphorus 0.0l (Maximum) the balance being iron and impurities.

In a preferred embodiment of the invention, the heating and cooling steps are repeated. It has been found that a maximum reduction in grain size is obtained when the heating and cooling cycles are repeated a total of three times. A coarse grained maraging steel treated in this manner has a grain size of ASTM No. 7 as determined by the method of ASTM El 12-63. Further heating and cooling cycles do not result in any additional reduction in grain size.

A better understanding of the invention can be obtained by referring to the following illustrative examples which are not intended to be unduly limitative of the invention. In the examples grain size determinations were made in accordance with the method of ASTM El l2-63.

EXAMPLE I Percent Nickel [9.95 Cobalt 9.53 Molybdenum 4.93 Titanium 0.69 Aluminum 0.1 l Carbon Silicon 0.05 Manganese 0.02 Sulfur 0.005 Phosphorus 0.005 lron Balance One specimen was heated in a tensile testing machine to 1,600 E, held at this temperature for minutes, and tested to indicate a yield strength of 47.5 [(51 (thousand pounds per square inch) and an ultimate tensile strength of 55.0 KSl. The second specimen was heated in the furnace to l,900 F., immediately cooled to 1.600 F., held for 15 minutes at this temperature and tested. The results for the second specimen were a yield strength of 13.0 [(8] and an ultimate tensile strength of 40.5 KSl.

The two specimens were cooled to room temperature and then subjected to a microstructural examination. The specimen heated to l,900 F. showed a finer grain size than the other specimen. The fact that a finer grained specimen had lower strength properties than a coarser grained specimen is what one would expect. However, it was unexpected that grain refinement actually occurred as a result of heating to a temperature of 1,900 E.

EXAMPLE II A test was conducted for the purpose of determining accurately the refining temperature for a 300-grade maraging steel. A Satec gradient furnace was used in this test. This furnace utilizes a specially designed muffle to create a uniform temperature gradient within the furnace chamber. The hot back wall can be set to any desired temperature up to 2,500 F. The temperature gradient to the front opening is determined from empirical charts. Any specimen inserted into the chamber necessarily exhibits a uniform temperature gradient. The temperature can be constantly monitored by thermocoupled attached to the specimen and feeding through a multiple switch to a recorder. A thermocouple affixed to the hot and cold ends of the specimen establishes the temperature profile which can then be dimensionally determined very accurately at any point.

in this test a coarse grained specimen in the form of a rod one-half inch in diameter and 7 inches long was heated in the furnace so that there existed along its length a thermal gradient from l,400 to 2,338 F. The rod was held in this temperature gradient for l hour after which it was water quenched. A microstructural examination of the cooled rod indicated that the ends of the rod had coarse grains with finer grains in the rods middle. The finest grains were found at a dimension corresponding to l,880 F. The grains gradually enlarged for about 1 inch on either side of the l,880 F. mark. The cooler end exhibited the original grain size while the hot end had experienced grain growth.

This test indicated that the optimum refining temperature for the 300-grade maraging steel specimen was 1,880 F.

EXAMPLE III A test was carried out to simulate the grain refinement of a large section of a 300-grade maraging steel. A coupon was heated in a large furnace from ambient temperature to l,880

F. The heating took several hours. After holding at l,880 F. for 10 minutes, the specimen was furnace cooled overnight. Microstructural examination of the specimen indicated that grain refinement had occurred.

This test indicated that heating and cooling rates had no appreciable effect upon grain refinement.

EXAMPLE IV A series of tests were conducted to demonstrate the effect of thermal cycling, i.e., repeated heating and cooling of a specimen. in the tests a 1-inch round ofa coarse grained, 300- grade maraging steel was heated to l,880 F., held for a few seconds at this temperature and then cooled to ambient temperature. Between cycles a disk was sectioned from the round for microexamination. As many as six cycles and examinations were performed on a single bar.

It was found that the grain size produced by one thermal refinement cycle, although finer than the original structure, was not as fine as could be achieved by using more than one cycle. Three cycles were required to produce the minimum grain size obtainable. Commencing with a specimen having a grain size of ASTM No. l to No. l, the grains were reduced in size on the first cycle to ASTM No. 4, on the second cycle to ASTM No. 5% and on the third cycle to ASTM No. 7. Additional cycling did not decrease the grain size. The ASTM No. 4 and No. 5% sizes were interspersed with finer gains while the ASTM No. 7 grains were uniform. It was also found during these tests that the specimen had to be cooled to below that temperature at which magnetic martensite transformation was completed or refinement did not occur. For the 300-grade maraging steel, this temperature was about 200 F.

EXAMPLE V A test was conducted to determine the temperature below which a 300-grade maraging steel must be cooled in order to obtain grain refinement. A rod-shaped specimen was heated in a furnace to the l,880 F. refining temperature. The heated specimen was thereafter cooled in a gradient furnace which was controlled so that a temperature gradient of to 400 F. was impressed on the rod. The specimen was cycled three times between the heating furnace and the gradient furnace after which it was allowed to air cool to room temperature. The transfers between furnaces were made as rapidly as possible.

Examination of the cooled bar indicated that the third of the bar corresponding to a temperature gradient of 100 to 200 F. had undergone uniform grain refinement. A gradual coarsening of the grain structure was evident above the 200 F. mark and up to about the 250 F. mark. The remainder of the bar representing temperatures to 400 F. exhibited the coarse grain structure of the original specimen.

It was previously known from the dilatometer trace of 300- grade maraging steel that martensite transformation upon cooling begins at 325 F. and is complete at 200 F. It is thus seen from the foregoing data that to obtain grain refinement it is necessary to cool a specimen to below that temperature at which magnetic martensite transformation is completed.

EXAMPLE VI A test was conducted in which a specimen of a 250-grade maraging steel was treated in accordance with the method of this invention. This steel had the following composition, expressed in weight percent:

Pcrccnt Nickel l8.l Cobalt 8.6 Molybdcnum 53 Titanium 0.4 Aluminum 0.05 Carbon 0J6 Silicon 0.05 Manganese 0.07

Chromium 0.05 Boron 0.002 Copper 0, I Zirconium 0.002 lron Balance The specimen had a grain size of ASTM No. 1. It was heated to l,700 F. after which it was cooled to 380 F. The heating and cooling cycles were repeated a total of five times. Examination of the treated specimen indicated that it had a grain size ofASTM No. 7.

The method of this invention is applicable to the treatment of any coarse grained maraging steel having a composition substantially as indicated hereinbefore. Various types of fabricating methods can be used in the production of end products from maraging steels. These methods include cutting, shearing, grinding, hot working, mechanical machining procedures. welding and heat treating. It often happens that the finished and semifinished products resulting from these fabrication methods have coarse grains because of overheating. By proceeding in accordance with the method of the present invention, such products can be thermally refined so that they have a finer gain structure. As a result the products are easier to work and they possess improved properties, such as improved stress corrosion life and high fracture toughness.

While the invention has been described with a certain degree of particularity, it is to be understood that modifications thereof can be made by one skilled in the art without departing from the spirit and scope of the invention.

I claim:

I. A method of refining a coarse grained maraging steel which comprises the steps of heating said steel having a martensite structure so that said steel is at a uniform temperature in the range of about l,700 to l,900 F., the steel at said temperature having an austenite structure; and cooling the resulting heated steel to a temperature in the range of about to 400 F., thereby completely transforming the austenite structure of the steel to a magnetic martensite structure, repeating the step of heating the cooled steel to a uniform temperature in the range of about l,700 to l,900 F. to obtain an austenite structure and subsequently cooling the steel to a temperature in the range of about 175 to 400 F to completely transform the austenite to a magnetic martensite structure, the resulting cooled steel product having a decreased grain size, said maraging steel having the following composition. expressed in weight percent:

Percent Nickel 15-26 Cobalt 7-l3 Molybdenum 2.75-5.5 Titanium 0.7-1.6 Aluminum obs-0.3 Carbon 0.03 (Maximum) Silicon 0.10 (Maximum) Manganese 0.l0 (Maximum) Sulfur 0.10 (Maximum) Phosphorus 0.l0 (Maximum) 

2. A method according to claim 1 in which said heating and colling steps are repeated a total of at least three times.
 3. A method according to claim 2 in which said coarse grained maraging steel has a grain size of ASTM No. -1 to No. 1 and said steel product has a grain size of ASTM No. 7, the grain sizes being determined in accordance with the method of ASTM E112-63. 